A. Technical Field
The present invention relates to a process for production of a particulate water-absorbent resin. Specifically, the present invention relates to a process for production of a particulate water-absorbent resin having been surface-treated by a specific surface-treatment process.
B. Background Art
Since the past, a water-absorbent resin is used as one of component materials of sanitary napkins, disposable diapers, or other sanitary materials which absorb body fluids. Known examples of such a water-absorbent resin include: hydrolyzed graft polymers of starch-acrylonitrile; neutralized graft polymers of starch-acrylic acid; saponified copolymers of vinyl acetate-acrylic acid esters; hydrolyzed copolymers of acrylonitrile or acrylamide, or crosslinked polymers of these hydrolyzed copolymers; and partially-neutralized and crosslinked poly(acrylic acids).
Examples of properties, which the water-absorbent resin is desired to have, include high absorption capacity, excellent absorption rate, high gel strength, and excellent suction force to suck up water from base materials. However, the relations between these properties do not necessarily show positive correlations. Particularly, the absorption capacity has contrary relations with such as the absorption rate, gel strength, and suction force. For example, if the absorption capacity is enhanced, then, when contacting with liquids, the water-absorbent resin unfavorably forms what are called “fisheyes” to thus hinder water from diffusing throughout the entire water-absorbent resin particles, with the unfavorable result that such as the absorption rate is extremely lowered.
As a process to prevent such a phenomenon to thus obtain a water-absorbent resin which exhibits a high absorption capacity and is comparatively good also in such as the absorption rate, for example, there is known a process in which surfaces of water-absorbent resin particles are coated with surfactants and/or nonvolatile hydrocarbons. However, as to this process, the dispersibility of water being initially absorbed may be improved, but no sufficient effects are obtained in point of the enhancement of the absorption rate and suction force of individual particles.
There are known processes in which surfaces of water-absorbent resins are treated with certain specific crosslinking agents to thereby enhance the crosslinking density of surface portions of the water-absorbent resins (e.g. refer to patent document 1 below).
Examples of the crosslinking agents used in these processes include polyhydric alcohols, polyglycidyl ethers, haloepoxy compounds, polyaldehydes, polyamines, and polyvalent metal salts. However, many of the crosslinking agents having often been used since the past have problems in the safety and the skin irritativity (e.g. polyglycidyl ethers). If it is taken into consideration that the surface-treated water-absorbent resin is used for the sanitary materials (e.g. diapers), then cases where such crosslinking agents remain on surfaces of the water-absorbent resin are much problematic in aspects such as safety. On the other hand, among the crosslinking agents having often been used since the past, there are also crosslinking agents which are less problematic in aspects such as safety (e.g. polyhydric alcohols). However, in cases where such crosslinking agents are used, their reactivity is so low that their reaction temperatures need to be made high (as the case may be, their reaction times need to be made long). In cases where the reaction temperatures are made high, there have been problems that much energy is needed because operations of raising and dropping the temperatures are also needed.
As surface treatment processes which can solve the above problems in the safety and the skin irritativity while the properties which the water-absorbent resin is desired to have are still kept on levels as high as possible, there are reported processes in which treating liquids containing the same monomers and crosslinking agents as monomers and crosslinking agents having been used for production of base polymers are used for surface treatment of the base polymers (e.g. refer to patent document 2 below).
However, problems have remained in that much energy is needed because the reaction temperatures for the surface treatment are still high.                [Patent Document 1]                    JP-A-044627/1982 (Kokai)                        [Patent Document 2]                    Japanese Patent No. 2530668                        